1. Field of the Invention
The present invention relates to a liquid level detecting apparatus for detecting the level or height of a liquid, and more particularly to a liquid level detecting apparatus in which a liquid level detecting resistor is placed in the liquid and the resistance of the detecting resistor varies in accordance with its length of immersed portion in the liquid.
2. Prior Art
As shown in FIG. 3, both a level detecting resistor Rd and a temperature compensating resistor of a conventional liquid level detecting apparatus take the form of a wire wound resistor that is wound around an elongated supporting member 3 or 8. The level detecting resistor Rd is placed vertically in the liquid as shown in FIG. 4 and is supplied a constant current Id from a constant current source 1. Likewise, the temperature compensating resistor Rc is inserted into the liquid as shown in FIG. 4. FIG. 2 shows a conventional liquid level detecting apparatus of this type. The voltage drop developed across the level detecting resistor Rd is supplied to an inverting input terminal (-) of an OP amplifier OP3 via the temperature compensating resistor Rc. The output of the OP amplifier OP3 is fed back to the inverting input terminal (-) thereof via a feedback resistor R6. In other words, the resistors Rc, R6, and the amplifier OP3 form a negative feedback amplifier whose gain is given by R6/Rc.
The resistors Rd and Rc have the following temperature characteristics. EQU Rd=Rdt(1+.alpha.d.DELTA.T)L (1) EQU Rc=Rct(1+.alpha.c.DELTA.T)L (2)
where Rdt and Rct are resistances per unit length of the level detecting resistor and temperature compensating resistor, respectively, at a certain ambient air temperature, for example, t=20.degree. C. Rd and Rc are the overall resistances of the level detecting resistor and temperature compensating resistor, respectively, when the ambient air temperature increases by an increment .DELTA.T from t.degree. C. .alpha.d and .alpha.c are the temperature coefficients of resistances of the level detecting resistor Rd and temperature compensating resistor Rc, respectively, at the ambient air temperature t.degree. C. L is the overall length of the respective resistors.
When the constant current Id flows through the detecting resistor Rd, the current Id generates an amount of heat to further increase the resistance of Rd at the temperature t.degree. C. This heat is radiated to the liquid from a portion immersed in the liquid, causing the overall resistance to decrease somewhat. Thus, it is understood that the overall resistance of the level detecting resistor Rd at the ambient temperature t.degree. C. varies with the length immersed in the liquid. In other words, the voltage developed across the level detecting resistor Rd represents the liquid level.
If .alpha.d=.alpha.c=.alpha. and both the level detecting resistor and temperature compensating resistor are inserted into the liquid, Eq. (1) and Eq. (2) are rewritten as follows: ##EQU1## where X is a length of each resistor immersed in the liquid. a is a constant specific to the liquid. K is a quantity given by K=I.sup.2 R.alpha.o.theta., that is determined by the temperature coefficient .alpha.o (.alpha.d or .alpha.c), the current I (Id or Ic) through the resistor R (Rd or Rc), and the thermal resistance .theta. in radiating heat generated therein to atmosphere . . . LS1 Therefore, ##EQU2## Thus, the output voltage Vout of the OP amplifier OP3 is given as follows: ##EQU3## If Id&gt;&gt;Ic, then the overall resistance of the temperature compensating resistor Rc does not vary with the length immersed in the liquid, thus ##EQU4##
Rdt and Rct are the resistances per unit length of the level detecting resistor Rd and the temperature compensating resistor Rc at a specific ambient air temperature, for example, t=20.degree. C., and therefore are fixed values. Now, Vout changes with the length X immersed in the liquid, not being affected by the change .DELTA.T in the ambient temperature.
With this prior art apparatus, since a relatively large current Id is run through the level detecting resistor Rd for higher sensitivity, the voltage developed thereacross is relatively high. As a result, the high voltage across the level detecting resistor Rd tends to cause a current to flow through the temperature compensating resistor Rc, the current through Rc generates an amount of heat which in turn causes the resistance of Rc to vary. The change in resistance of Rc due to self-generated heat will result in deviation from the designed temperature versus resistance curve of the level detecting apparatus. To prevent this kind of measurement error, generally the temperature compensating resistor Rc is selected to be a large value so to as to maintain as low a current through Rc as possible. A large value of Rc requires more turns of wire and/or smaller diameters of the wire, which causes more complicated manufacture process. If Rd and Rc differ in diameter of the wire, they differ in temperture coefficient .alpha.. This causes a problem in temperature compensation of the level detecting resistor. Excess turns of wire that are wound around the supporting member tend to increase heat capacity of the resistor Rc, which takes a longer time for the resistance to settle when the ambient temperature changes, causing error in temperature compensation. In addition, the level detecting apparatus cannot follow a rapid change in liquid level.